a. EV-DO Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1×RTT networks” (or “1× networks” for short), which stands for “Single Carrier Radio Transmission Technology.” These networks typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Recently, service providers have introduced mobile stations and wireless networks that communicate using a protocol known as EV-DO, which stands for “Evolution Data Optimized.” EV-DO networks, operating in conformance with industry specification IS-856, provide high rate packet-data service (including Voice over IP (VoIP) service) to mobile stations using a combination of time-division multiplexing (TDM) on the forward link (from the network to mobile stations) and CDMA technology on the reverse link (from mobile stations to the network). Furthermore, some mobile stations, known as hybrid mobile stations or hybrid wireless access terminals, can communicate with both 1× networks and EV-DO networks.
In the EV-DO context, a mobile station is typically referred to as a wireless access terminal, while the network entity with which the wireless access terminal communicates over the air interface is known as a wireless access node. The wireless access node typically includes a device known as a radio network controller (RNC), which is similar to a base station controller (BSC) in 1× networks. The wireless access node also includes one or more base transceiver stations (BTSs), each of which includes one or more antennas that radiate to define respective wireless coverage areas. Among other functions, the RNC controls one or more BTSs, and acts as a conduit between the BTSs and an entity known as a packet data serving node (PDSN), which provides access to a packet-data network. Thus, when positioned in one of these wireless coverage areas, a wireless access terminal may communicate over the packet-data network via the wireless access node and the PDSN.
b. Reverse Noise Rise
Interference can be—and often is—present in a wireless network coverage area. In general, on a given carrier in a given sector of a wireless coverage area, a wireless access node will receive transmissions from wireless access terminals that are operating on that carrier in that sector. However, the wireless access node often also receives transmissions on that carrier from other wireless access terminals, other devices, and/or any other sources of interference on that frequency. At a given moment, the sum total of what a wireless access node is receiving on a given carrier is known as the “reverse noise” on that carrier.
At any time, and in fact quite frequently (e.g., once for every forward-link timeslot (i.e. once every approximately 1.67 ms)), wireless access nodes compute a value known as “reverse noise rise” (RNR), which is the difference between (i) the reverse noise that the wireless access node is currently detecting and (ii) a baseline level of reverse noise. Thus, the wireless access node computes how far the reverse noise has risen above that baseline.
To determine the baseline, EV-DO networks typically periodically utilize what is known as a silent interval, which may occur on the order of once every five minutes, and last on the order of 40-100 ms, both of which are typically configurable. During the silent interval, wireless access terminals know not to transmit anything to the wireless access node. The wireless access node can then measure whatever else is out there. As such, the baseline corresponds to the amount of reverse noise on the carrier in the sector when the sector is unloaded (i.e. without any wireless access terminals). Note that other reverse-link-noise levels could be used as a baseline, other than the unloaded-sector level.
In general, the lower the RNR is at a given moment, the more favorable the RF environment is for communication between wireless access terminals and the wireless access node at that moment. Correspondingly, the higher the RNR, the less favorable the RF environment is. Also, a low RNR generally corresponds to a carrier in a sector being lightly loaded (i.e., supporting communications for a relatively low number of wireless access terminals). In contrast, a high RNR generally corresponds to a carrier in a sector being heavily loaded (i.e., supporting communications for a relatively high number of wireless access terminals).
c. Reverse Activity Bit
Wireless access nodes typically use the calculated value of RNR to, among other things, set what is known as the Reverse Activity Bit (RAB), which is a value that the wireless access node sets to 0 or 1, and repeatedly transmits to all the wireless access terminals operating on a given carrier in a given sector in a wireless coverage area. As stated above, the wireless access node typically calculates RNR at the same frequency at which it transmits forward-link timeslots, or once every 1.67 ms. The wireless access node typically maintains or changes the value of the RAB at this same frequency.
With respect to how the wireless access node chooses to set the RAB to 0 or 1, if the RNR is at or above a reverse-activity threshold, which may be about 5 dB, the wireless access node sets the RAB to 1. If, on the other hand, the RNR is less than the reverse-activity threshold, the wireless access node sets the RAB to 0. The wireless access node then transmits the RAB in a TDM channel—known as the reverse-activity channel—on the forward link. The reverse activity channel is itself a TDM portion of a forward-link channel known as the Media Access Control (MAC) channel. Note that the RAB is the same for all wireless access terminals on a given carrier in a given sector.